The present invention relates to a tandem press line that includes a plurality of presses and a plurality of transfer systems that are alternately disposed in a line direction, and is configured so that each press can be subjected to synchronized press control using an individual press control signal that corresponds to and is synchronized with a line master control signal, and each transfer system can be subjected, to synchronized transfer control using an individual transfer control signal that corresponds to and is synchronized with the line master control signal.
A tandem press line has been known in which a plurality of presses and a plurality of transfer systems are alternately disposed in a line direction. A workpiece is transferred to the first press from a material feeder that is disposed on the upstream side in the workpiece transfer direction, sequentially transferred to the intermediate presses, and then press-formed by the final press to obtain a product. The product is transferred to a product handling system that is disposed on the downstream side of the final press. Note that the workpiece transferred to the first press is a material. A semifinished product is obtained by each intermediate press, and a product is obtained by the final press.
Such a tandem press line normally employs an alternate/intermittent operation method. Specifically, each transfer system is operated in one cycle in a state in which each press is stopped at the upper dead point (top dead center). Each transfer system removes the workpiece from the preceding-stage (upstream-side) press, and transfers the workpiece to the subsequent-stage (downstream-side) press. A transfer member of each transfer system that has transferred the workpiece stands by at a position at which interference with the press does not occur. Each press is then operated in one cycle so that the workpiece is press-formed. Each press is then stopped at the upper dead point.
The alternate/intermittent operation method has an advantage in that interference (e.g., collision) between the element of the press and the element of the transfer system can be reliably prevented, since the press and the transfer system are alternately operated. However, the alternate/intermittent operation method has a disadvantage in that productivity decreases due to wastage of time.
In order to deal with the above problem, a press line in which a press and a transfer system are operated in parallel has been proposed (see WO2004/096533A1, for example). The press line is configured so that the speed of a motor of the downstream-side press is controlled so that the downstream-side press follows the upstream-side press, and the difference in crank angle between the upstream-side press and the downstream-side press is constant. The transfer system is controlled to follow the operation of the upstream-side press and the operation of the downstream-side press, and is independently controlled at the intermediate stage (i.e., follow-target-switching parallel operation method). The follow-target-switching parallel operation method is considered, to improve productivity due to a reduction in wastage of time while preventing interference as compared with the alternate/intermittent operation method.
However, the follow-target-switching parallel operation method aims at reducing facing wear by eliminating clutch operation and braking (i.e., reducing the maintenance cost and the maintenance frequency). Therefore, the follow-target-switching parallel operation method is not advantageous for a servo press line that is not provided with a clutch, a brake, and a flywheel.
Specifically, the upstream-side press and the downstream-side press must have a relative relationship with a sufficient, allowance in terms of time when determining the relative relationship between the upstream-side press and the downstream-side press (that can be controlled at high speed) in advance, and controlling the operation of the transfer system that has a large mechanical inertia and is controlled at a relatively low speed so that the transfer system follows the operation of the upstream-side press and the operation of the downstream-side press. This makes it difficult to achieve a significant improvement in productivity. Moreover, the difference in crank angle between the upstream-side press and the downstream-side press can be made constant only when the slide motion of the upstream-side press and the slide motion of the downstream-side press are identical. Specifically, it is meaningless to achieve a constant difference in crank angle when a press line is configured so that a slide motion optimum for each press-forming operation is set for each press.
It is necessary to improve the productivity of the entire press line instead, of merely improving local productivity (e.g., adjacent presses). For example, JP-A-2008-246529 discloses an integrally controlled press line that is configured so that a host controller generates a press motion parameter that optimizes the operation of each press and maximizes the operation speed of each press, a transfer motion parameter that optimizes the operation of each transfer system and maximizes the operation speed of each transfer system, and a parallel operation phase signal. The host controller outputs the press motion parameter, the transfer motion parameter, and the phase signal to a plurality of sub-controllers, and each press and each transfer system are operated using a signal from each sub-controller. Specifically, each press is subjected to synchronized press control using an individual press control signal that corresponds to and is synchronized with a line master control signal, and each transfer system is subjected to synchronized transfer control using an individual transfer control signal that corresponds to and is synchronized with the line master control signal. Since each press and each transfer system can be integrally controlled so that the performance of each press and each transfer system can be maximized while ensuring the press-forming accuracy of each press and preventing interference, productivity can be significantly improved.
An integrally controlled press line that includes an individual stop control device has been proposed (see JP-A-2009-172662, for example). Specifically, the press or the transfer system in which an abnormality has occurred is stopped. The press or the transfer system that may interfere with the press or the transfer system in which an abnormality has occurred is stopped at a position at which interference can be prevented, and the press or the transfer system that is normally operated is stopped at a normal stop position. This prevents emergency shutdown of the entire press line. This makes it possible to prevent a situation in which a press that is normally operated produces a defective product.
The method, disclosed in JP-A-2009-172662 can prevent a situation in which the press that is normally operated produces a defective product, but sequentially stops the press or the transfer system in which an abnormality has occurred, the press or the transfer system that may interfere with the press or the transfer system in which an abnormality has occurred, and the press or the transfer system that is normally operated. Specifically, the method disclosed in JP-A-2009-172662 stops the entire press line in the same manner as emergency shutdown. This concept is contradictory to the concept of the method disclosed in WO2004/096533A1 that eliminates the disadvantage of the alternate/intermittent operation method, and is also contradictory to the concept that aims at improving productivity. The stop position of each press and each transfer system varies depending on the operation state and the like when an abnormality has occurred. This means that it takes time to resume the operation. This also results in a decrease in productivity.
An abnormality may necessarily occur when operating the press line due to mechanical, electrical, or human error. On the other hand, an improvement in productivity and quality (e.g., press-forming accuracy) is strongly desired. Therefore, it is desirable to avoid stopping the entire press line even if an abnormality has occurred in order to prevent a significant decrease in productivity. For example, it may be possible to eliminate an abnormality when temporarily reducing the operation speed.
However, when the line operation speed is reduced, in order to remove an abnormality, the press-forming accuracy may deteriorate although a significant decrease in productivity does not occur as compared with the case of stopping the entire press line. The production cost decreases due to occurrence of a defective product with a low press-forming accuracy, so that tangible productivity decreases. Therefore, development of a tandem press line that makes it possible to eliminate the cause of an abnormality without stopping the entire press line, allows continuous operation, and can maintain and improve tangible productivity, has been desired.